INTRODUCTION Libraries frequently support their open access (OA) fund using money from their collections budget. Interest in assessment of OA funds is arising. Cost per use is a common method to assess library collections expenditures. OA article processing charges (APCs) are a one-time cost for global, perpetual use. Article level metrics provide data on global, cumulative article level usage. This article examines a method and discusses the limitations and implications of using article level metrics to calculate cost per use for OA APCs. METHODS Using different APC models from two publishers, PLOS and BioMed Central, this article presents a cost per use formula for each model. RESULTS The formula for each model is demonstrated with available data. The examples suggest a very low cost per use for OA APCs after only three years. DISCUSSION Several limitations exist to obtaining article level data currently, including the nature of open access and accessibility of the data. OA articles’ usage levels are high and include use from altruistic access. Cost per use comparison with traditional publishing models is possible; however, comparison between different OA expenditures with very low costs per use may not be helpful. CONCLUSION Article level metrics can provide a means to measure cost per use of OA APCs. Libraries need increased access to article level usage data. They will also need to develop new benchmarks and expectations to evaluate APC payments, given higher usage levels for OA articles and considering altruistic access.